1. Field of the Invention
The present invention relates to an image formation method of developing a latent electrostatic image to a visible image using a toner which comprises a binder resin, a coloring agent and a charge controlling agent comprising a quaternary ammonium salt.
2. Discussion of Background
In conventional image formation apparatus, such as electrophotographic copying machines, printers and facsimile apparatus, in which latent electrostatic images are formed on a latent-electrostatic-image bearing member and developed to visible images by a developer, dry type development units using a powder-like developer are widely used.
As such powder-like developers, a two-component type developer comprising a toner and a carrier, and a one-component type developer comprising a toner without containing a carrier, are conventionally known.
A two-component type development method using the above two-component type developer is capable of yielding relatively stable, good recorded images, but has the shortcomings that the deterioration of the carrier is easily caused, and the mixing ratio of the toner and the carrier tends to change while in use, so that the maintenance of an apparatus using this method is complicated. Furthermore, the apparatus using the two-component type development method is relatively oversized.
From the above viewpoints, the primary focus of attention is on a one-component type development method using the one-component type developer, which does not have the above-mentioned shortcomings of the two-component type development method.
There are two types of one-component type developers. One is of a type which consists of a toner, while the other is of a type which consists of a mixture of a toner and an auxiliary agent.
Furthermore, there are two types of toners. One is a magnetic toner which contains magnetic particles, and the other is a non-magnetic toner which does not contain magnetic particles.
Generally, the above-mentioned magnetic particles are not transparent. Therefore, it is difficult to obtain clear color images including full-color images and multi-colored images by use of the magnetic toner. Therefore, it is preferable to employ the one-component type development method using the non-magnetic toner when the color images are to be obtained.
In U.S. patent application Ser. No. 597,881 (filed Nov. 12, 1990) and Ser. No. 674,161 (filed Mar. 25, 1991), Suzuki et al. disclose an image formation method in which a one-component type developer comprising a non-magnetic toner, when necessary with addition of auxiliary agents thereto, is supplied to the surface of a developer-bearing member which is rotatably driven to transport the developer into a development zone, where a latent-electrostatic-image-bearing member is directed to the above-mentioned developer-bearing member, so that the latent electrostatic images formed on the latent-electrostatic-image-bearing member are developed to visible images, characterized in that numerous micro closed electric fields are formed near the surface of the developer-bearing member by selectively causing the surface of the developer-bearing member to support electric charges, the charged toner is attracted by these closed electric fields to deposit the developer on the surface of the developer-bearing member, thereby developing the latent electrostatic images to visible toner images.
This method has many advantages over the conventional methods, including the advantage that the intensity of the electric fields can be significantly increased in comparison with the case where the conventional methods are employed, since a number of micro closed electric fields are formed near the surface of the developer-bearing member, and therefore a large amount of sufficiently charged toner can be deposited on the developer-bearing member and transported into the development zone.
According to the inventors of the present invention, however, in the above image formation method of forming numerous micro fields near the surface of the developer-bearing member, it is difficult to form a toner layer with a uniform thickness on the developer-bearing member in a stable manner when a conventional non-magnetic toner comprising a coloring agent and a resin is employed for the development of a latent-electrostatic image. The toner layer formed on the developer-bearing member becomes thin and accordingly the amount of the toner to be used for development is decreased. As a result, the following problems occur: The density of the obtained images is low, the toner layer is not uniform, and the images and the background are fogged. Furthermore, a thin film of toner is formed on the developer-bearing member, which is a so-called filming phenomenon. When this phenomenon takes place, the effect of the micro fields is reduced, and eventually the amount of the toner held by the developer-bearing member is decreased and it becomes difficult to supply a sufficient amount of the toner to the latent-electrostatic-image bearing member.